The industrial, scientific, and medical (“ISM”) radio band with a central frequency of 915 MHz is a widely used frequency band in the U.S. for home area networking products (specifically for home automation and smart grid applications). Wireless products using this band are typically subjected to severe interference from other radio products, such as baby monitors, cordless phones, IEEE 802.15.4 radios, etc. These radio products, especially baby monitors, may use this frequency band to transmit analog video signal which can, individually, easily occupy approximately or up to 70% of the entire band. If another home area network device is using any frequency occupied by the analog baby monitor, that device will encounter channel access failure.
Most modern devices implement a radio technology in the form of a Carrier Sense Multiple Access (“CSMA”) Collision Avoidance (“CSMA-CA”) mechanism. For example, a device may sense the channel before it sends any packets and back off if the channel is busy. Mitigating and/or avoiding interference caused by other radio devices in general is a challenging problem. The 915 MHz frequency band in particular is challenging because many of the existing devices in this radio band use an aggressive channel access scheme and occupy a large percentage of the entire band. CSMA is an example of a channel access scheme that is not aggressive. Other similar radio technologies may use a “listen before talk” protocol that, prior to attempting transmission of a packet, sets a radio to receive mode first. The radio may attempt to listen on the channel to determine if any other device is transmitting at that point. If it determines another device is transmitting on the channel, then it may not transmit or wait for a predetermined amount of time before attempting transmission. An aggressive scheme may refer to an instance where a device is constantly transmitting, such as a baby monitor.
Substantial research has been invested in interference mitigation for 802.15.4 devices, e.g., coexistence of 802.15.4 (ZigBee) and 802.11 (WiFi) devices since they often co-locate in home area networks and both use the same 2.4 GHz frequency band. Due to the differences in transmit power and bit rate, a ZigBee device can suffer significantly from WiFi interference. A popular approach to solve the coexistence problem is to adjust the Clear Channel Assessment (“CCA”) threshold in 802.15.4 to make its channel access scheme more/less aggressive. This approach is not particularly useful in the 915 MHz, ISM frequency band since many of the devices responsible for the interference do not apply a CSMA scheme (e.g., a baby monitor).
ZigBee defines a frequency agility mechanism in which all devices keep a counter of channel access failure. When that counter passes a given threshold (e.g., 25%), all devices enter a discovery mode. Each device first performs an energy scan of all channels and then sends the scan result back to the network manager device. The network manager determines if the entire network should switch to a different channel and, if so, to which channel the entire network should switch. Thus, it uses the already interfered channel to communicate the scan result and channel switch command. These messages can easily be lost because the channel already exhibits a high level of interference. This may cause the entire frequency agility scheme to be unreliable.